1. Field of Invention
The present invention relates to dense fluid cleaning. More particularly to the use of a dense fluid such as carbon dioxide in a process of centrifugal phase shifting separation as an environmentally sound alternative to organic solvents.
2. Description of Related Art
Liquid-phase phase carbon dioxide cleaning devices and processes can be found in the art. However, none utilize a dense fluid centrifuge or centrifugal processes similar to those detailed herein.
Carbon dioxide exists as a low-density gas at standard temperature and pressure conditions and possesses phase boundaries with a triple point (Solid-Liquid-Gas co-exist in equilibrium like a glass of ice cubes and water) and a critical point (Liquid-Gas have identical molar volumes). Through pressure or temperature modification, carbon dioxide can be compressed into a dense gas state.
Compressing carbon dioxide at a temperature below its critical temperature (C.T.) liquefies the gas at approximately 70 atm. Cooling liquid-state or gas-state carbon dioxide to its freezing point causes a phase transition into solid-state carbon dioxide. Compressing carbon dioxide at or above its critical temperature and critical pressure (C.P.) also increases its density to a liquid-like state (5), however there is a significant difference between compression below and above the critical point.
Compressing carbon dioxide above its critical point does not effect a phase change. In fact, carbon dioxide at a temperature at or above 305 K (88 F.) cannot be liquefied at any pressure, yet the density for the gas may be liquid-like. At the critical point the density is approximately 0.47 g/ml. At or above this point carbon dioxide is termed a supercritical fluid (SCF). Supercritical carbon dioxide can be compressed to a range of liquid-like densities, yet it will retain the diffusivity of a gas. Continued compression of supercritical carbon dioxide causes continued increase in density, approaching that of its liquid phase.
Carbon dioxide is but one of the compounds which is adequate for use as the dense fluid in the within invention other compounds exhibiting suitable dense fluid properties include propane, butane, sulfur hexafluoride liquid nitrogen and liquid ammonia. Those skilled in the art will recognize that without exceeding the intended scope of this invention other compounds exhibiting similar dense fluid properties may be substituted for use in the herein described apparatus and processes.
The use of conventional dense fluid cleaning technology may result in a pooling and supercooling of liquid carbon dioxide trapped within pores and cavities of a substrate—leading to the formation of dry ice and recontamination of substrate. The process and apparatus to perform dense fluid centrifugal separations described herein results in precision cleaning using one unit volume of dense fluid per cleaning operation and ability to remove small insoluble particles from deep voids or cavities without dry ice formation or recontamination of the substrate. Accordingly, the within dense fluid cleaning and separation apparatus and process overcomes limitations of conventional dense fluid technology and may provide an environmentally-safer cleaning and finishing alternative to organic solvents.